Fully double-ended operational amplifiers typically include an input stage and a transimpedance stage, which may be the output stage. The transimpedance stage may be simply a pair of single-ended transimpedance signal branches which are identical to each other, with each receiving one of the signals from the input stage. An example of a fully differential monolithic operational amplifier implemented in complementary metal-oxide-silicon (CMOS) technology is described in "High-Frequency CMOS Switched-Capacitor Filters for Communications Application" by T. C. Choi et al. in IEE Journal of Solid State Circuits, vol. SC-18, No. 6, Dec. 1983, pp. 652-664. In the arrangement described therein the output stage is made up of a parallel pair of mutually identical folded cascode signal branches, connected between two supply voltage nodes.
The performance of the amplifier depends in large part on a high open loop gain achieved by means of a high output impedance for the signal branches. There is a tradeoff in these characteristics. If the cascode transistors in the signal branches are made with a relatively low width-to-length ratio conduction channel for increased gain, then the output impedance of the branches is reduced accordingly.